The present invention relates generally to systems and methods for controlling the temperature of compressed gas fuels such as compressed natural gas (CNG) and Hydrogen (H2), and for controlling the temperature and vaporization of liquified compressed gas fuels such as liquified natural gas (LNG), propane, and liquefied petroleum gas (LPG). More particularly, the present invention relates to controlling the temperature of compressed gas fuel within a fuel supply system by using a gaseous heat transfer medium to transfer heat from the exhaust of an internal combustion engine to the fuel supply system.
A typical compressed gas fuel supply system includes a fuel container, such as a metallic tank, containing the fuel under pressure, and a valve at one end to access the contents thereof. The fuel container is filled with a compressed gas fuel, which may be liquified. In systems where the compressed gas fuel is liquefied, the liquified compressed gas fuel is vaporized under ambient heat to provide an operating pressure under which the vaporized fuel is withdrawn through the tank valve. As vapor is withdrawn, the remaining liquid vaporizes and, in doing so, absorbs an amount of heat known as the latent heat of vaporization. This causes the temperature of the remaining liquid to decrease, which in turn reduces its ability to vaporize. Due to continued withdrawal of the vapor, the pressure of the system continues to fall, and the liquid temperature continues to decline. If the fuel temperature is permitted to drop below freezing, various elements of the fuel supply system, such as control devices, may not function properly. Since heat absorbed during vaporization of the liquid must be replenished from heat in the environment surrounding the fuel container, the temperature and vapor pressure inside the fuel container will decrease below functional limits unless sufficient heat is available in the environment surrounding the fuel container.
Like liquified compressed gas fuel systems, gas fuel supply systems utilizing compressed gas fuel that is not liquiefied also absorb heat as the fuel flows through the system. Compressed gas fuel systems absorb heat when the gas is expanded to lower the gas pressure below the tank pressure, as in pressure regulators. Typically, less heat is absorbed when expanding a compressed gas than when vaporizing a liquified compressed gas. Nonetheless, gas fuel supply systems that expand compressed gas fuel must be supplied with heat to avoid excessive temperature and pressure drops, as explained above with reference to liquified compressed gas fuel supply systems.
The temperature of the vaporizing liquid in the fuel container can be raised or maintained by supplying heat to the fuel container from the environment surrounding the fuel container. In home heating applications, for instance, tanks with large surface areas for heat transfer are used with low vapor flow rates, so sufficient heat may be available in the ambient atmosphere. In large stationary applications such as power plants, vaporization heat may be supplied by exposing the tank to a large body of water, such as a lake or ocean. But in mobile applications, such as in motor vehicles powered by compressed gas fuel (e.g., hybrid electric transit buses powered indirectly by turbines run on propane), large tanks or large heat sinks are not practical.
Motor vehicles powered by compressed gas fuel have used the freely available waste heat from the liquid engine coolant to ensure fuel vaporization and fuel temperature. In existing high pressure gaseous fuel systems, hot liquid engine coolant is circulated through a small coolant-to-fuel heat exchanger to add heat to the fuel, preventing freezing of the fuel control components. The heat exchanger is typically added to the fuel system at the exit of a pressure regulator, or may be incorporated into the pressure regulator itself. A typical system is disclosed in U.S. Pat. No. 5,540,208, wherein a compressed gas fuel supply system is used to supply compressed gas fuel to an internal combustion engine of an automobile. The liquid-phase gas fuel is vaporized by means of a heat exchanger, using liquid as the heat transfer medium.
A disadvantage of using liquid, such as engine coolant, as the heat transfer medium is that liquid engine coolant often requires a significant amount of time to reach normal operating temperature. During this delay, insufficient heat may be available to properly vaporize the compressed gas fuel. Other disadvantages of using liquid as the heat transfer medium are that such systems tend to leak, tend to corrode the heat exchanger, and require regular maintenance. Finally, air-cooled engines, including some turbine engines, do not use liquid engine coolant, and, thus, expensive retrofitting would be required to utilize liquid as the heat transfer medium.
An aspect of the present invention involves a system and method for transferring heat from the exhaust of an internal combustion engine to a fuel container storing compressed gas fuel, using a gaseous heat transfer medium. The heat transferred to the compressed gas fuel container assists expansion of compressed gas fuel, and assists vaporization of liquified compressed gas fuel. The expanded and/or vaporized compressed gas fuel may be delivered to the internal combustion engine for use by the engine.
Another aspect of the invention involves a compressed gas fuel supply system for use with an exhaust system of an internal combustion engine. The compressed gas fuel supply system includes a container with a compressed gas fuel that may be liquified, an exhaust heat transfer system, and a gaseous heat transfer medium that circulates through the exhaust heat transfer system. The exhaust heat transfer system is in thermal communication with the exhaust system of the internal combustion engine for heat transfer from the exhaust system to the gaseous heat transfer medium. The exhaust heat transfer system is also in thermal communication with the fuel container for heat transfer from the gaseous heat transfer medium to the fuel container for expanding the compressed gas fuel, and/or at least partially vaporizing liquified compressed gas fuel.
Another aspect of the invention involves a method of expanding compressed gas fuel and/or vaporizing liquified compressed gas fuel from a fuel container, wherein the fuel container is part of a compressed gas fuel supply system for use with an exhaust system of an internal combustion engine. The method includes providing an exhaust heat transfer system in thermal communication with the exhaust system of the internal combustion engine and the fuel container, the exhaust heat transfer system including a gaseous heat transfer medium that circulates therethrough; transferring heat from the exhaust heat transfer system to the gaseous heat transfer medium circulating through the exhaust heat transfer system; and transferring heat from the gaseous heat transfer medium circulating through exhaust heat transfer system to the compressed gas fuel in the fuel container so as to assist said compressed gas fuel to expand, and/or to assist said liquified compressed gas fuel to at least partially vaporize.
An additional aspect of the invention involves a compressed gas fuel supply system comprising a pressure regulator for expanding the compressed gas and lowering its pressure, and a fuel container that may be a small finned heat exchanger. The fuel container is placed directly after the pressure regulator. The invention further involves an exhaust heat transfer system, and a gaseous heat transfer medium that circulates through the exhaust heat transfer system. The exhaust heat transfer system is in thermal communication with the exhaust system of an internal combustion engine for heat transfer from the exhaust system to the gaseous heat transfer medium. The exhaust heat transfer system is also in thermal communication with the fuel container for heat transfer from the gaseous heat transfer medium to the fuel container to add heat to the compressed gas fuel as it expands through the pressure drop created by the pressure regulator.
A further aspect of the invention involves an exhaust heat transfer system for use with a fuel container including a compressed gas fuel and/or a liquified compressed gas fuel, and an exhaust system of an internal combustion engine. The exhaust heat transfer system includes an exhaust heat transfer member in thermal communication with the exhaust system of the internal combustion engine, a heat exchanger in thermal communication with the fuel container, and one or more connection members connecting the heat exchanger and the exhaust heat transfer member for circulating a gaseous heat transfer medium therethrough to remove heat from the exhaust system and add heat to the fuel container for expansion of the compressed gas fuel, and/or for vaporization of the liquified compressed gas fuel.
Other and further objects, features, aspects, and advantages of the present invention will become better understood with the following detailed description of the accompanying drawings.